1. Field of the Invention
The present invention relates to a solid state radiation detector with a storage portion for storing a quantity of electric charge proportional to a quantity of radiation irradiated or quantity of light emitted by excitation of the radiation, as latent image charge.
2. Description of the Related Art
Today, in the field of radiation photography with the object of medical analysis, etc., a wide variety of radiation image information recording-reading units have been proposed and put to practical use. The recording-reading unit uses a solid state radiation detector or static storage medium (also stated as simply a detector), which temporarily stores electric charge obtained by detecting radiation, as latent image charge in its charge storage portion and also converts the stored latent image charge to an electrical signal representing radiation image information and outputs the converted signal.
Various types have been proposed as solid state radiation detectors that are used in the recording-reading unit. For instance, there is an optical reading type which employs the process of reading out a stored electric charge from the detector. In this type of detector, the stored electric charge is read out by irradiating reading light (e.g., electromagnetic waves for reading) to the detector.
The applicant of this application has proposed, in Japanese Patent Application Nos. 10 (1998)-271374, 11 (1999)-87922, and 11 (1999)-89553 published as Japanese Unexamined Patent Publication Nos. 2000-162726. 2000-284056, and 2000-284057, respectively, a solid state radiation detector of an optical reading type in which high-speed reading responsivity is compatible with efficient fetching of signal charge from the detector. The detector is constructed of (1) a first electrode layer (conductive layer) which has permeability with respect to recording radiation, or light emitted by excitation of the radiation (hereinafter referred to as recording radiation, etc.), (2) a recording photoconductive layer which exhibits electric conduction when irradiated with the recording light, etc., (3) a charge transfer layer which operates as substantially an insulator with respect to an electric charge of the same polarity as electric charge on the first electrode layer and also operates as substantially an electric conductor with respect to an electric charge of the opposite polarity, (4) a reading photoconductive layer which exhibits electric conduction when irradiated with reading light (electromagnetic waves for reading), and (5) a second electrode layer (conductive layer) which has permeability with respect to the reading light, which are stacked in the recited order. In this type of detector, signal charge (latent image charge) carrying image information is stored in a charge storage portion formed in the interface between the recording photoconductive layer and the charge transfer layer.
Particularly, in the above-mentioned Japanese Patent Application Nos. 11 (1999)-87922 and 11 (1999)-89553, there is proposed a detector where the electrode (light irradiating electrode) of a second conductive layer having permeability with respect to reading light is constructed with a stripe electrode consisting of a large number of main line electrodes. Also, a great number of secondary line electrodes, for outputting an electric signal which has a level proportional to a quantity of latent image charge stored in the charge storage portion, are provided within the second conductive layer so that the main and secondary line electrodes are alternately arranged in parallel to one another.
Thus, by providing the charge fetching electrode which consists of secondary line electrodes, within the second electrode layer, an additional capacitor is formed between the charge storage portion and the secondary line electrodes, and a transfer charge of the opposite polarity from the latent image charge stored in the charge storage portion by recording can be transferred to the secondary line electrodes by charge rearrangement at the time of reading. This can make smaller the quantity of the aforementioned transfer charge distributed to the capacitor formed between the main line electrodes and the charge storage portion through the reading photoconductive layer, compared with the case where the secondary line electrodes are not provided. As a result, the quantity of signal charge that can be fetched from the detector is made larger and therefore the fetch efficiency is enhanced. In addition, high-speed reading responsivity is compatible with efficient fetching of signal charge.
However, in the case where the-transmission factor of each main line electrode of the stripe electrode with respect to the reading light is small, or the case where the transmission factor of each secondary line electrode of the charge fetching electrode with respect to the reading light is great, even if the secondary line electrodes are provided within the second electrode layer, there is a possibility that a quantity of signal charge that can be fetched from the detector will become smaller. In addition, the quantity of signal charge that can be fetched from the detector varies depending on the area of the main or secondary line electrodes.
The present invention has been made in view of the aforementioned drawbacks found in the prior art. Accordingly, it is the primary object of the present invention to provide a solid state radiation detector which is capable of reliably making larger a quantity of signal charge that can be fetched therefrom.
The inventors of this application, in the detectors disclosed in the aforementioned [publication] Japanese Patent Application No. 11 (1999)-87922, particularly the detector where the main line electrodes and the secondary line electrodes are provided in the secondary electrode layer so that the main and secondary line electrodes are alternately arranged in parallel to one another, have made various investigations and experiments with respect to the relationship between the transmission factors and areas of the main and secondary line electrodes with respect to reading light and the magnitude of a quantity of signal charge that can be fetched from the detector, and have found the following relationship therebetween.
(1) The quantity of signal charge that can be fetched from the detector becomes larger, if the total quantity (quantity of light transmitted) R1 of the reading light incident on the reading photoconductive layer through the main line electrodes forming the stripe electrode for light irradiation is larger and also the total quantity R2 of the reading light incident on the reading photoconductive layer through the secondary line electrodes is smaller, that is, if the ratio R1/R2 of the total light quantity R1 of the former to the total light quantity R2 of the latter is greater.
Note that in the case where the distance between the main line electrode, for light irradiation and the secondary line electrode is not negligible with respect to the electrode width, there is a need to take this distance between electrodes into consideration. However, the space between electrodes is normally set small and filled with a material which intercepts the reading light. Therefore, the influence of the space on the quantity of signal charge is considered practically negligible.
(2) The total quantity of the reading light incident on the reading photoconductive layer through the electrodes is proportional to the product of the areas of the electrodes and the transmission factor with respect to the reading light, if the irradiation intensity of the reading light is the same. Since the length of the main line electrode for light irradiation is essentially the same as that of the secondary line electrode, the total quantity of the reading light incident on the reading photoconductive layer through the electrodes is considered practically proportional to the product of the widths of the electrodes and the transmission factor. That is, it is considered that R1 equals Wbxc3x97Pb and R2 equals Wcxc3x97Pc.
(3) Therefore, both the transmission factor of each electrode with respect to the reading light and the electrode width need to be considered in order to reliably make larger a quantity of signal charge that can be fetched from the detector. If at least the ratio R1/R2 of the total light quantities is 1 or greater, it is considered that a sufficient quantity of signal charge can be obtained even when the transmission factor of the main line electrode with respect to the reading light is, for example, about 50%.
The present invention has been made based on the aforementioned new knowledge. That is, a solid state radiation detector according to the present invention comprises a first electrode layer having permeability with respect to recording radiation, or light emitted by excitation of the radiation; a recording photoconductive layer which exhibits electric conduction when irradiated with the recording radiation or the light; a reading photoconductive layer which exhibits electric conduction when irradiated with reading light; and a second electrode layer constructed of a large number of main line electrodes having permeability with respect to the reading light. The first electrode layer, the recording photoconductive layer, the reading photoconductive layer, and the second electrode layer are stacked in the recited order. A large number of secondary line electrodes, for outputting an electrical signal which has a level proportional to a quantity of latent image charge stored in a charge storage portion formed between the recording photoconductive layer and the reading photoconductive layer, are provided within the second electrode layer so that the main and secondary line electrodes are alternately arranged in parallel to one another. The width Wb of the main line electrode, the transmission factor Pb of the main line electrode with respect to the reading light, the width Wc of the secondary line electrode, and the transmission factor Pc of the secondary line electrode with respect to the reading light satisfy the following condition equation (1):
(Wbxc3x97Pb)/(Wcxc3x97Pc)xe2x89xa71xe2x80x83xe2x80x83(1)
The above-mentioned condition equation (1) means that the total quantity (quantity of light transmitted) of the reading light incident on the reading photoconductive layer through the main line electrodes is always larger than the total quantity (quantity of light transmitted) of the reading light incident on the reading photoconductive layer through the secondary line electrodes, in spite of the electrode widths and transmission factors of the main and secondary line electrodes, and also in spite of the quantity of the reading light.
Note that it is desirable that the right side of the equation be 5, and more desirable that it be 8. Furthermore, it is desirable that the right side of the equation be 12.
In the case where a plurality of main and secondary line electrodes are allocated to 1 pixel, preferably the ratio of the product of the width and transmission factor of the main line electrode per pixel and the product of the width and transmission factor of the secondary line electrode per pixel is determined so that it satisfies the above-mentioned condition equation. For instance, in the case where the transmission factors of the main line electrodes are all the same and also the transmission factors of the secondary line electrodes are all the same, the sum total (Wb) of the widths of the main line electrodes and the sum total (Wc) of the widths of the secondary line electrodes are set so that they satisfy the above-mentioned condition equation. Also, in the case where the transmission factors of the main line electrodes differ from one another, the case where the transmission factors of the secondary line electrodes differ from one another, and furthermore, the case where the number of main line electrodes differs from that of secondary line electrodes, the product of the width and transmission factor of each main line electrode in 1 pixel and the product of the width and transmission factor of each secondary line electrode in 1 pixel are calculated and then the ratio of the total sums is set so that it satisfies the above-mentioned equation (1). This can be represented by the following condition equation (2):                                           WP            b                                WP            c                          =                                                            ∑                                  i                  =                  1                                m                            ⁢                              xe2x80x83                            ⁢                                                W                  bi                                xc3x97                                  W                  bi                                                                                    ∑                                  j                  =                  1                                n                            ⁢                              xe2x80x83                            ⁢                                                W                  cj                                xc3x97                                  P                  cj                                                              ≥          1                                    (        2        )            
in which WPb is the product of the width and transmission factor of the main line electrode per pixel, WPc is the product of the width and transmission factor of the secondary line electrode per pixel, m is the number of main line electrodes per pixel, Wbi is the width of each main line electrode, Pbi is the transmission factor of each main line electrode, n is the number of secondary line electrodes per pixel, Wcj is the width of each secondary line electrode, and Pcj is the transmission factor of each secondary line electrode.
As with the aforementioned condition (1), it is desirable that the right side of the equation be 5, and more desirable that it be 8. Furthermore, it is desirable that the right side of the equation be 12.
To satisfy the above-mentioned condition (1) or (2), it is preferable that the material of the main line electrode for light irradiation be any one among indium tin oxide (ITO), Idemitsu indium X-metal oxide (IDIXO, produced by Idemitsu Kosan), aluminum, and molybdenum, and it is preferable that the material of the secondary line electrode be any one among aluminum, molybdenum, and chrome.
The expression xe2x80x9ccharge storage portion formed between the recording photoconductive layer and the reading photoconductive layerxe2x80x9d as used herein and in the appended claims is intended to mean a charge storage portion for storing a quantity of electric charge, generated within the recording photoconductive layer when irradiated with radiation carrying image information or irradiated with light emitted by excitation of the radiation, which is proportional to the quantity of the radiation or quantity of the light emitted by excitation of the radiation.
The method of forming the charge storage portion may employ, for example, a method of forming a charge storage portion in the interface between a charge transfer layer and a recording photoconductive layer (see the aforementioned Japanese Patent Application Nos. 10 (1998)-27137 and 11 (1999)-87922, filed by the applicant of this application), a method of forming a charge storage portion within a trapping layer or in the interface between the trapping layer and a recording photoconductive layer (see U.S. Pat. No. 4,535,468), or a method of providing micro conductive members on which latent image charge is concentrated (see the aforementioned Japanese Patent Application No. 11 (1999)-89553, filed by the applicant of this application).
Note that when recording or reading out a radiation image by the use of the detector of the present invention, a conventional recording and reading method and a unit thereof can be utilized as they are, without changing them.
The present invention has been made based on the new knowledge on the relationship between the transmission factors and areas of the main and secondary line electrodes with respect to the reading light and the quantity of signal charge that can be fetched from the detector, and in consideration of both the transmission factor of each electrode with respect to the reading light and the width of the main line electrode in order to reliably make larger a quantity of signal charge that can be fetched from the detector, the width Wb of the mainline electrode, the transmission factor Pb of the main line electrode with respect to the reading light, the width Wc of the secondary line electrode, and the transmission factor Pc of the secondary line electrode with respect to the reading light are set so that they satisfy the aforementioned condition equation (1). Therefore, regardless of the sizes of Wc and Wb, the detector of the present invention is capable of reliably making larger a quantity of signal charge that can be fetched therefrom and reliably enhancing the fetch efficiency and the image signal-to-noise (S/N) ratio.
In addition, if the ratio of the product of the width and transmission factor of the main line electrode per pixel and the product of the width and transmission factor of the secondary line electrode per pixel is set so that it satisfies the aforementioned condition equation (2), even in the case where a plurality of main line electrodes and a plurality of secondary line electrodes are allocated to 1 pixel, a quantity of signal charge that can be fetched from the detector can be reliably made larger, even if there are fluctuations in the widths and transmission factors of the main and secondary line electrodes.